Chapter 10. Loops and Branches

 

What needs this iteration, woman?

--Shakespeare, Othello

Operations on code blocks are the key to structured and organized shell scripts. Looping and branching constructs provide the tools for accomplishing this.

10.1. Loops

A loop is a block of code that iterates [1] a list of commands as long as the loop control condition is true.

for loops

for arg in [list]

This is the basic looping construct. It differs significantly from its C counterpart.

for arg in [list]
do
command(s)...
done

Note

During each pass through the loop, arg takes on the value of each successive variable in the list.

   1 for arg in "$var1" "$var2" "$var3" ... "$varN"  
   2 # In pass 1 of the loop, arg = $var1	    
   3 # In pass 2 of the loop, arg = $var2	    
   4 # In pass 3 of the loop, arg = $var3	    
   5 # ...
   6 # In pass N of the loop, arg = $varN
   7 
   8 # Arguments in [list] quoted to prevent possible word splitting.

The argument list may contain wild cards.

If do is on same line as for, there needs to be a semicolon after list.

for arg in [list] ; do


Example 10-1. Simple for loops

   1 #!/bin/bash
   2 # Listing the planets.
   3 
   4 for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
   5 do
   6   echo $planet  # Each planet on a separate line.
   7 done
   8 
   9 echo; echo
  10 
  11 for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
  12     # All planets on same line.
  13     # Entire 'list' enclosed in quotes creates a single variable.
  14     # Why? Whitespace incorporated into the variable.
  15 do
  16   echo $planet
  17 done
  18 
  19 echo; echo "Whoops! Pluto is no longer a planet!"
  20 
  21 exit 0

Each [list] element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see Example 14-16) to force parsing of each [list] element and assignment of each component to the positional parameters.


Example 10-2. for loop with two parameters in each [list] element

   1 #!/bin/bash
   2 # Planets revisited.
   3 
   4 # Associate the name of each planet with its distance from the sun.
   5 
   6 for planet in "Mercury 36" "Venus 67" "Earth 93"  "Mars 142" "Jupiter 483"
   7 do
   8   set -- $planet  #  Parses variable "planet"
   9                   #+ and sets positional parameters.
  10   #  The "--" prevents nasty surprises if $planet is null or
  11   #+ begins with a dash.
  12 
  13   #  May need to save original positional parameters,
  14   #+ since they get overwritten.
  15   #  One way of doing this is to use an array,
  16   #         original_params=("$@")
  17 
  18   echo "$1		$2,000,000 miles from the sun"
  19   #-------two  tabs---concatenate zeroes onto parameter $2
  20 done
  21 
  22 # (Thanks, S.C., for additional clarification.)
  23 
  24 exit 0

A variable may supply the [list] in a for loop.


Example 10-3. Fileinfo: operating on a file list contained in a variable

   1 #!/bin/bash
   2 # fileinfo.sh
   3 
   4 FILES="/usr/sbin/accept
   5 /usr/sbin/pwck
   6 /usr/sbin/chroot
   7 /usr/bin/fakefile
   8 /sbin/badblocks
   9 /sbin/ypbind"     # List of files you are curious about.
  10                   # Threw in a dummy file, /usr/bin/fakefile.
  11 
  12 echo
  13 
  14 for file in $FILES
  15 do
  16 
  17   if [ ! -e "$file" ]       # Check if file exists.
  18   then
  19     echo "$file does not exist."; echo
  20     continue                # On to next.
  21    fi
  22 
  23   ls -l $file | awk '{ print $9 "         file size: " $5 }'  # Print 2 fields.
  24   whatis `basename $file`   # File info.
  25   # Note that the whatis database needs to have been set up for this to work.
  26   # To do this, as root run /usr/bin/makewhatis.
  27   echo
  28 done  
  29 
  30 exit 0

If the [list] in a for loop contains wild cards (* and ?) used in filename expansion, then globbing takes place.


Example 10-4. Operating on files with a for loop

   1 #!/bin/bash
   2 # list-glob.sh: Generating [list] in a for-loop, using "globbing"
   3 
   4 echo
   5 
   6 for file in *
   7 #           ^  Bash performs filename expansion
   8 #+             on expressions that globbing recognizes.
   9 do
  10   ls -l "$file"  # Lists all files in $PWD (current directory).
  11   #  Recall that the wild card character "*" matches every filename,
  12   #+ however, in "globbing," it doesn't match dot-files.
  13 
  14   #  If the pattern matches no file, it is expanded to itself.
  15   #  To prevent this, set the nullglob option
  16   #+   (shopt -s nullglob).
  17   #  Thanks, S.C.
  18 done
  19 
  20 echo; echo
  21 
  22 for file in [jx]*
  23 do
  24   rm -f $file    # Removes only files beginning with "j" or "x" in $PWD.
  25   echo "Removed file \"$file\"".
  26 done
  27 
  28 echo
  29 
  30 exit 0

Omitting the in [list] part of a for loop causes the loop to operate on $@ -- the positional parameters. A particularly clever illustration of this is Example A-16. See also Example 14-17.


Example 10-5. Missing in [list] in a for loop

   1 #!/bin/bash
   2 
   3 #  Invoke this script both with and without arguments,
   4 #+ and see what happens.
   5 
   6 for a
   7 do
   8  echo -n "$a "
   9 done
  10 
  11 #  The 'in list' missing, therefore the loop operates on '$@'
  12 #+ (command-line argument list, including whitespace).
  13 
  14 echo
  15 
  16 exit 0

It is possible to use command substitution to generate the [list] in a for loop. See also Example 15-54, Example 10-10 and Example 15-48.


Example 10-6. Generating the [list] in a for loop with command substitution

   1 #!/bin/bash
   2 #  for-loopcmd.sh: for-loop with [list]
   3 #+ generated by command substitution.
   4 
   5 NUMBERS="9 7 3 8 37.53"
   6 
   7 for number in `echo $NUMBERS`  # for number in 9 7 3 8 37.53
   8 do
   9   echo -n "$number "
  10 done
  11 
  12 echo 
  13 exit 0

Here is a somewhat more complex example of using command substitution to create the [list].


Example 10-7. A grep replacement for binary files

   1 #!/bin/bash
   2 # bin-grep.sh: Locates matching strings in a binary file.
   3 
   4 # A "grep" replacement for binary files.
   5 # Similar effect to "grep -a"
   6 
   7 E_BADARGS=65
   8 E_NOFILE=66
   9 
  10 if [ $# -ne 2 ]
  11 then
  12   echo "Usage: `basename $0` search_string filename"
  13   exit $E_BADARGS
  14 fi
  15 
  16 if [ ! -f "$2" ]
  17 then
  18   echo "File \"$2\" does not exist."
  19   exit $E_NOFILE
  20 fi  
  21 
  22 
  23 IFS=$'\012'       # Per suggestion of Anton Filippov.
  24                   # was:  IFS="\n"
  25 for word in $( strings "$2" | grep "$1" )
  26 # The "strings" command lists strings in binary files.
  27 # Output then piped to "grep", which tests for desired string.
  28 do
  29   echo $word
  30 done
  31 
  32 # As S.C. points out, lines 23 - 30 could be replaced with the simpler
  33 #    strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'
  34 
  35 
  36 #  Try something like  "./bin-grep.sh mem /bin/ls"
  37 #+ to exercise this script.
  38 
  39 exit 0

More of the same.


Example 10-8. Listing all users on the system

   1 #!/bin/bash
   2 # userlist.sh
   3 
   4 PASSWORD_FILE=/etc/passwd
   5 n=1           # User number
   6 
   7 for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" )
   8 # Field separator = :    ^^^^^^
   9 # Print first field              ^^^^^^^^
  10 # Get input from password file               ^^^^^^^^^^^^^^^^^
  11 do
  12   echo "USER #$n = $name"
  13   let "n += 1"
  14 done  
  15 
  16 
  17 # USER #1 = root
  18 # USER #2 = bin
  19 # USER #3 = daemon
  20 # ...
  21 # USER #30 = bozo
  22 
  23 exit 0
  24 
  25 #  Exercise:
  26 #  --------
  27 #  How is it that an ordinary user (or a script run by same)
  28 #+ can read /etc/passwd?
  29 #  Isn't this a security hole? Why or why not?

Yet another example of the [list] resulting from command substitution.


Example 10-9. Checking all the binaries in a directory for authorship

   1 #!/bin/bash
   2 # findstring.sh:
   3 # Find a particular string in the binaries in a specified directory.
   4 
   5 directory=/usr/bin/
   6 fstring="Free Software Foundation"  # See which files come from the FSF.
   7 
   8 for file in $( find $directory -type f -name '*' | sort )
   9 do
  10   strings -f $file | grep "$fstring" | sed -e "s%$directory%%"
  11   #  In the "sed" expression,
  12   #+ it is necessary to substitute for the normal "/" delimiter
  13   #+ because "/" happens to be one of the characters filtered out.
  14   #  Failure to do so gives an error message. (Try it.)
  15 done  
  16 
  17 exit $?
  18 
  19 #  Exercise (easy):
  20 #  ---------------
  21 #  Convert this script to take command-line parameters
  22 #+ for $directory and $fstring.

A final example of [list] / command substitution, but this time the "command" is a function.

   1 generate_list ()
   2 {
   3   echo "one two three"
   4 }
   5 
   6 for word in $(generate_list)  # Let "word" grab output of function.
   7 do
   8   echo "$word"
   9 done
  10 
  11 # one
  12 # two
  13 # three

The output of a for loop may be piped to a command or commands.


Example 10-10. Listing the symbolic links in a directory

   1 #!/bin/bash
   2 # symlinks.sh: Lists symbolic links in a directory.
   3 
   4 
   5 directory=${1-`pwd`}
   6 #  Defaults to current working directory,
   7 #+ if not otherwise specified.
   8 #  Equivalent to code block below.
   9 # ----------------------------------------------------------
  10 # ARGS=1                 # Expect one command-line argument.
  11 #
  12 # if [ $# -ne "$ARGS" ]  # If not 1 arg...
  13 # then
  14 #   directory=`pwd`      # current working directory
  15 # else
  16 #   directory=$1
  17 # fi
  18 # ----------------------------------------------------------
  19 
  20 echo "symbolic links in directory \"$directory\""
  21 
  22 for file in "$( find $directory -type l )"   # -type l = symbolic links
  23 do
  24   echo "$file"
  25 done | sort                                  # Otherwise file list is unsorted.
  26 #  Strictly speaking, a loop isn't really necessary here,
  27 #+ since the output of the "find" command is expanded into a single word.
  28 #  However, it's easy to understand and illustrative this way.
  29 
  30 #  As Dominik 'Aeneas' Schnitzer points out,
  31 #+ failing to quote  $( find $directory -type l )
  32 #+ will choke on filenames with embedded whitespace.
  33 #  Even this will only pick up the first field of each argument.
  34 
  35 exit 0
  36 
  37 
  38 # --------------------------------------------------------
  39 # Jean Helou proposes the following alternative:
  40 
  41 echo "symbolic links in directory \"$directory\""
  42 # Backup of the current IFS. One can never be too cautious.
  43 OLDIFS=$IFS
  44 IFS=:
  45 
  46 for file in $(find $directory -type l -printf "%p$IFS")
  47 do     #                              ^^^^^^^^^^^^^^^^
  48        echo "$file"
  49        done|sort
  50 
  51 # And, James "Mike" Conley suggests modifying Helou's code thusly:
  52 
  53 OLDIFS=$IFS
  54 IFS='' # Null IFS means no word breaks
  55 for file in $( find $directory -type l )
  56 do
  57   echo $file
  58   done | sort
  59 
  60 #  This works in the "pathological" case of a directory name having
  61 #+ an embedded colon.
  62 #  "This also fixes the pathological case of the directory name having
  63 #+  a colon (or space in earlier example) as well."
  64 

The stdout of a loop may be redirected to a file, as this slight modification to the previous example shows.


Example 10-11. Symbolic links in a directory, saved to a file

   1 #!/bin/bash
   2 # symlinks.sh: Lists symbolic links in a directory.
   3 
   4 OUTFILE=symlinks.list                         # save file
   5 
   6 directory=${1-`pwd`}
   7 #  Defaults to current working directory,
   8 #+ if not otherwise specified.
   9 
  10 
  11 echo "symbolic links in directory \"$directory\"" > "$OUTFILE"
  12 echo "---------------------------" >> "$OUTFILE"
  13 
  14 for file in "$( find $directory -type l )"    # -type l = symbolic links
  15 do
  16   echo "$file"
  17 done | sort >> "$OUTFILE"                     # stdout of loop
  18 #           ^^^^^^^^^^^^^                       redirected to save file.
  19 
  20 exit 0

There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses.


Example 10-12. A C-style for loop

   1 #!/bin/bash
   2 # Multiple ways to count up to 10.
   3 
   4 echo
   5 
   6 # Standard syntax.
   7 for a in 1 2 3 4 5 6 7 8 9 10
   8 do
   9   echo -n "$a "
  10 done  
  11 
  12 echo; echo
  13 
  14 # +==========================================+
  15 
  16 # Using "seq" ...
  17 for a in `seq 10`
  18 do
  19   echo -n "$a "
  20 done  
  21 
  22 echo; echo
  23 
  24 # +==========================================+
  25 
  26 # Using brace expansion ...
  27 # Bash, version 3+.
  28 for a in {1..10}
  29 do
  30   echo -n "$a "
  31 done  
  32 
  33 echo; echo
  34 
  35 # +==========================================+
  36 
  37 # Now, let's do the same, using C-like syntax.
  38 
  39 LIMIT=10
  40 
  41 for ((a=1; a <= LIMIT ; a++))  # Double parentheses, and "LIMIT" with no "$".
  42 do
  43   echo -n "$a "
  44 done                           # A construct borrowed from 'ksh93'.
  45 
  46 echo; echo
  47 
  48 # +=========================================================================+
  49 
  50 # Let's use the C "comma operator" to increment two variables simultaneously.
  51 
  52 for ((a=1, b=1; a <= LIMIT ; a++, b++))
  53 do  # The comma chains together operations.
  54   echo -n "$a-$b "
  55 done
  56 
  57 echo; echo
  58 
  59 exit 0

See also Example 26-16, Example 26-17, and Example A-6.

---

Now, a for loop used in a "real-life" context.


Example 10-13. Using efax in batch mode

   1 #!/bin/bash
   2 # Faxing (must have 'efax' package installed).
   3 
   4 EXPECTED_ARGS=2
   5 E_BADARGS=85
   6 MODEM_PORT="/dev/ttyS2"   # May be different on your machine.
   7 #                ^^^^^      PCMCIA modem card default port.
   8 
   9 if [ $# -ne $EXPECTED_ARGS ]
  10 # Check for proper number of command-line args.
  11 then
  12    echo "Usage: `basename $0` phone# text-file"
  13    exit $E_BADARGS
  14 fi
  15 
  16 
  17 if [ ! -f "$2" ]
  18 then
  19   echo "File $2 is not a text file."
  20   #     File is not a regular file, or does not exist.
  21   exit $E_BADARGS
  22 fi
  23   
  24 
  25 fax make $2              #  Create fax-formatted files from text files.
  26 
  27 for file in $(ls $2.0*)  #  Concatenate the converted files.
  28                          #  Uses wild card (filename "globbing")
  29 			 #+ in variable list.
  30 do
  31   fil="$fil $file"
  32 done  
  33 
  34 efax -d "$MODEM_PORT"  -t "T$1" $fil   # Finally, do the work.
  35 # Trying adding  -o1  if above line fails.
  36 
  37 
  38 #  As S.C. points out, the for-loop can be eliminated with
  39 #     efax -d /dev/ttyS2 -o1 -t "T$1" $2.0*
  40 #+ but it's not quite as instructive [grin].
  41 
  42 exit $?   # Also, efax sends diagnostic messages to stdout.

while

This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand.

while [ condition ]
do
command(s)...
done

The bracket construct in a while loop is nothing more than our old friend, the test brackets used in an if/then test. In fact, a while loop can legally use the more versatile double-brackets construct (while [[ condition ]]).

As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

while [ condition ] ; do

Note that the test brackets are not mandatory in a while loop. See, for example, the getopts construct.


Example 10-14. Simple while loop

   1 #!/bin/bash
   2 
   3 var0=0
   4 LIMIT=10
   5 
   6 while [ "$var0" -lt "$LIMIT" ]
   7 #      ^                    ^
   8 # Spaces, because these are "test-brackets" . . .
   9 do
  10   echo -n "$var0 "        # -n suppresses newline.
  11   #             ^           Space, to separate printed out numbers.
  12 
  13   var0=`expr $var0 + 1`   # var0=$(($var0+1))  also works.
  14                           # var0=$((var0 + 1)) also works.
  15                           # let "var0 += 1"    also works.
  16 done                      # Various other methods also work.
  17 
  18 echo
  19 
  20 exit 0


Example 10-15. Another while loop

   1 #!/bin/bash
   2 
   3 echo
   4                                # Equivalent to:
   5 while [ "$var1" != "end" ]     # while test "$var1" != "end"
   6 do
   7   echo "Input variable #1 (end to exit) "
   8   read var1                    # Not 'read $var1' (why?).
   9   echo "variable #1 = $var1"   # Need quotes because of "#" . . .
  10   # If input is 'end', echoes it here.
  11   # Does not test for termination condition until top of loop.
  12   echo
  13 done  
  14 
  15 exit 0

A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however.


Example 10-16. while loop with multiple conditions

   1 #!/bin/bash
   2 
   3 var1=unset
   4 previous=$var1
   5 
   6 while echo "previous-variable = $previous"
   7       echo
   8       previous=$var1
   9       [ "$var1" != end ] # Keeps track of what $var1 was previously.
  10       # Four conditions on "while", but only last one controls loop.
  11       # The *last* exit status is the one that counts.
  12 do
  13 echo "Input variable #1 (end to exit) "
  14   read var1
  15   echo "variable #1 = $var1"
  16 done  
  17 
  18 # Try to figure out how this all works.
  19 # It's a wee bit tricky.
  20 
  21 exit 0

As with a for loop, a while loop may employ C-style syntax by using the double-parentheses construct (see also Example 9-33).


Example 10-17. C-style syntax in a while loop

   1 #!/bin/bash
   2 # wh-loopc.sh: Count to 10 in a "while" loop.
   3 
   4 LIMIT=10
   5 a=1
   6 
   7 while [ "$a" -le $LIMIT ]
   8 do
   9   echo -n "$a "
  10   let "a+=1"
  11 done           # No surprises, so far.
  12 
  13 echo; echo
  14 
  15 # +=================================================================+
  16 
  17 # Now, repeat with C-like syntax.
  18 
  19 ((a = 1))      # a=1
  20 # Double parentheses permit space when setting a variable, as in C.
  21 
  22 while (( a <= LIMIT ))   # Double parentheses, and no "$" preceding variables.
  23 do
  24   echo -n "$a "
  25   ((a += 1))   # let "a+=1"
  26   # Yes, indeed.
  27   # Double parentheses permit incrementing a variable with C-like syntax.
  28 done
  29 
  30 echo
  31 
  32 # C programmers can feel right at home in Bash.
  33 
  34 exit 0

Inside its test brackets, a while loop can call a function.
   1 t=0
   2 
   3 condition ()
   4 {
   5   ((t++))
   6 
   7   if [ $t -lt 5 ]
   8   then
   9     return 0  # true
  10   else
  11     return 1  # false
  12   fi
  13 }
  14 
  15 while condition
  16 #     ^^^^^^^^^
  17 #     Function call -- four loop iterations.
  18 do
  19   echo "Still going: t = $t"
  20 done
  21 
  22 # Still going: t = 1
  23 # Still going: t = 2
  24 # Still going: t = 3
  25 # Still going: t = 4

By coupling the power of the read command with a while loop, we get the handy while read construct, useful for reading and parsing files.

   1 cat $filename |   # Supply input from a file.
   2 while read line   # As long as there is another line to read ...
   3 do
   4   ...
   5 done
   6 
   7 # =========== Snippet from "sd.sh" example script ========== #
   8 
   9   while read value   # Read one data point at a time.
  10   do
  11     rt=$(echo "scale=$SC; $rt + $value" | bc)
  12     (( ct++ ))
  13   done
  14 
  15   am=$(echo "scale=$SC; $rt / $ct" | bc)
  16 
  17   echo $am; return $ct   # This function "returns" TWO values!
  18   #  Caution: This little trick will not work if $ct > 255!
  19   #  To handle a larger number of data points,
  20   #+ simply comment out the "return $ct" above.
  21 } <"$datafile"   # Feed in data file.

Note

A while loop may have its stdin redirected to a file by a < at its end.

A while loop may have its stdin supplied by a pipe.

until

This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is false (opposite of while loop).

until [ condition-is-true ]
do
command(s)...
done

Note that an until loop tests for the terminating condition at the top of the loop, differing from a similar construct in some programming languages.

As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

until [ condition-is-true ] ; do


Example 10-18. until loop

   1 #!/bin/bash
   2 
   3 END_CONDITION=end
   4 
   5 until [ "$var1" = "$END_CONDITION" ]
   6 # Tests condition here, at top of loop.
   7 do
   8   echo "Input variable #1 "
   9   echo "($END_CONDITION to exit)"
  10   read var1
  11   echo "variable #1 = $var1"
  12   echo
  13 done  
  14 
  15 # ------------------------------------------- #
  16 
  17 #  As with "for" and "while" loops,
  18 #+ an "until" loop permits C-like test constructs.
  19 
  20 LIMIT=10
  21 var=0
  22 
  23 until (( var > LIMIT ))
  24 do  # ^^ ^     ^     ^^   No brackets, no $ prefixing variables.
  25   echo -n "$var "
  26   (( var++ ))
  27 done    # 0 1 2 3 4 5 6 7 8 9 10 
  28 
  29 
  30 exit 0

How to choose between a for loop or a while loop or until loop? In C, you would typically use a for loop when the number of loop iterations is known beforehand. With Bash, however, the situation is fuzzier. The Bash for loop is more loosely structured and more flexible than its equivalent in other languages. Therefore, feel free to use whatever type of loop gets the job done in the simplest way.

Notes

[1]

Iteration: Repeated execution of a command or group of commands, usually -- but not always, while a given condition holds, or until a given condition is met.

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